The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to methods and systems for controlling depth profiles of marine geophysical sensor streamers as they are towed in a body of water.
Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water floor. Energy that interacts with interfaces, generally at the boundaries between layers of rock formations, is returned toward the surface and is detected by sensors on the one or more streamers. The detected energy is used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
For certain types of surveying, it can be important that a streamer is maintained as close as possible to a selected depth profile in the water. For example, it may be important in electromagnetic surveying to increase the towing depth with an optimum depth being as close as possible to the seafloor while keeping the streamer as level as possible. This towing arrangement should reduce noise originating from towing the streamer through the water. Devices commonly used to regulate streamer depth include lateral force and depth (LFD) control devices. In one example, the LFD control devices may be equipped with variable-incidence wings that are rotably fixed onto the streamer. When used in conjunction with pressure sensors capable of generating a signal related to depth, the LFD control devices may be used to regulate streamer depth. Hydrodynamic depressors may also be deployed to increase the towing depth of the streamer. For instance, hydrodynamic depressors may be deployed on the spreader lines, for example, to provide downward thrust as the streamer is towed through the water, thereby forcing down the forward ends of the streamers.
Some survey conditions necessitate towing of streamers over a seafloor that has a varying topography. While LFD control devices and hydrodynamic devices may be sufficient to maintain streamer depth for a flat seafloor, these devices will typically not allow active control of the streamer depth profile to follow a seafloor with a varying topography. Accordingly, the towing depth of the streamers may be limited by safety margins at the shallowest area of the survey area. This limitation is especially problematic in areas with a sloping seafloor or where equipment or natural formations extend above the seafloor.
Accordingly, there is a need for improved methods and systems for controlling depth in electromagnetic surveys, for example, to track the seafloor.